But what if roads were in three dimensions, not two? The highway interchange is the closest we get to layering transportation, and although these can get pretty complex, they're usually clustered at key nodes. In dense cities, stacking transport interchanges isn't really an option. Future city projections have traditionally taken transportation into three dimensions. Eugene Henard's Cities of the Future, a paper given in 1910 (and reproduced on John W.Reps' Urban Planning 1794-1918 site), suggested layering cityscapes, using elaborate cross-sections to show how space could be increased if transport and services were stacked. According to Henard, 'all the evil [of today's city] arises from the old traditional idea that "the bottom of the road must be on a level with the ground in its original condition." But there is nothing to justify such an erroneous view'.

Central to Henard's vision was the widespread adoption of the cement flat roof (a good decade before it became a key feature of the emerging Modernism): 'With all the varied advantages which the employment of armoured cement offers, the covering-in of our houses with a level platform has become a simple matter, and this platform could be planted with small flower gardens or adorned with verdure clad trellises.' This would, he felt, be a perfect stepping stone to an inevitable technical development: 'But a still more important function to be performed by these terraces is that in the near future they will be used as landing stages for aeroplanes. We have not as yet arrived at that point because up to the present the aviator has not gained sufficient mastery over his machine: but as man has at length succeeded in imitating the flight of the bird it is by no means improbable that he will eventually succeed in imitating the flight of the insect.'

It was a fantasy ahead of its time (influenced, in part, by H.G.Wells' The War in the Air, with its airships and bird-like contraptions. Wells much of the distinction between European machines and their Eastern equivalents, described as 'strange steeds [that] the engineering of Europe had begotten upon the artistic inspiration of Japan, came a long string of Asiatic swordsman. The wings flapped jerkily, click, block, clitter clock...'). Henard's vision encompassed architecture, too, as he imaged how cities would have to erect towers, up to 500m tall, for navigation purposes (he cited the importance of church spires to the early aerial navigators).

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Suffice to say that the dream of the three-dimensional city is still very much alive. In their new book, Skycar City, Dutch architecture firm MVRDV, together with students from the University of Wisconsin-Milwaukee School of Architecture and Urban Planning, propose a 'pre-emptive history,' an attempt to define how the metropolis of tomorrow would appear if, and it's a big if, the technological dream of the flying car was finally mastered. Splicing a timeline of emerging (if perpetually stalling) technologies like the Moller Skycar, with the science fiction visions of Syd Mead, George Lucas, Fritz Lang, etc. etc., the team behind the book posit a future of vertical construction around the giant tubes formed by the aerial routes, along which semi-automated craft zip relentlessly, from tiny sky bikes and sky Vespas, up to larger vehicles. Envisioning new typologies for everything from parking garages (below) to stadiums, the team's work is an experiment to see how far transportation can go towards shaping architecture.

MVRDV are adept at mixing theory with practice. Without compromising the quality of their built work, the firm has published several monographs and stand-alone projects (like Container City, 2002) that explore the role of density in modern life, and potential - often highly politically charged or deadpan ironic - methods of abating the crisis of space, like the vertical Pig City, or the cantilevered WOZOCO housing, or even the stacked landscape of their EXPO 2000 pavilion, a 'mini-ecosystem' that 'saves space, energy, time, water and infrastructure.'

On one level, Skycar City is a supreme piece of informed science fiction, an extrapolation of what we would do to embrace a seductive piece of imagined technology. On the other hand, it's also a way of trying to arrive at a place that already exists in our imaginations; the sci-fi metropolis with its swarming skies and three-dimensional, roller-coaster streets. These are cities familiar from Metropolis, Blade Runner, and The Fifth Element, by artists like Eric Hanson, carefully built up using models and now digital models, with the future literally pasted over the past. Skycar City envisions a world that will be shaped by modernity's accretions, leaving the original architecture beneath a 'city of canyons and a look of coral'. In this vision, there is no chance of being located away from the road, for within access to the transport network, you are stuck in one place, embedded in perpetual transportation. That which does not move, dies:

'Year 2210: The parts of the city that atrophy in darkness and isolation eventually fall into ruin; this includes most of the ancient 20th century structures holding onto addresses at ground level. Quality of life still dominates the city's organization: what was dark or decaying is discarded, and space not served directly by skycars is abandoned.'

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Some other things. Stephen Zacks has an extensive story on Dubai in the current issue of Metropolis, entitled 'Beyond the Spectacle', in which he implies that New York will be considered an antique city in a century's time, 'a place to visit for the sake of nostalgia... .somehow like how we think of Paris.' Its place will be taken by Dubai, where some 310 billion dollars has been spent on construction in ten years. A place of social, political and architectural hybridity, where Western firms can indulge their computer-generated fantasies and a veneer of liberalism cloaks an oppressive state.